Baryons: strongly interacting particles made of three quarks, such as the proton and the neutron from which atomic nuclei are made. Baryons carry most of the mass of ordinary matter, since the proton and neutron masses are nearly two thousand times higher than the electron mass. Electrons and neutrinos are called leptons and are subject to only the electromagnetic, gravitational, and weak interactions.
Big Bang: the moment in time when the expansion of the Universe started. We cannot reliably extrapolate our history before the Big Bang because the densities of matter and radiation diverge at that time. A transition through the Big Bang could be described only by a future theory that will unify quantum mechanics and gravity.
Blackbody radiation: the radiation obtained in complete thermal equilibrium with matter of some fixed temperature. The intensity of the radiation as a function of photon wavelength is prescribed by the Planck spectrum. The best experimental confirmation of this spectrum was obtained by the COBE satellite measurement of the cosmic microwave background (CMB).
Black hole: a region surrounded by an event horizon from which no particle (including light) can escape. A black hole